This invention relates to optical fiber waveguiding structures and, more particularly to the design and fabrication of such structures with reduced modal dispersion.
It is well known that waveguides supportive of many guided modes suffer from modal dispersion. A pulse of wave energy launched into a multimode waveguide excites many modes, each traveling at a different group velocity. At the output end of the waveguide, the pulse is spread out in time by an amount that is proportional to the length of the guide because of the different group velocities of each mode. This pulse spreading, or dispersion, can severely limit the information-carrying capacity of a waveguide communication system, with the magnitude of the problem increasing with the length of the waveguide links.
In U.S. Pat. No. 3,687,614, it is shown that modal dispersion in multimode waveguides can be reduced by deliberately enhancing the mode coupling processes in the waveguide. Such mode coupling is accomplished by introducing controlled "imperfections" into the waveguide which tend to induce coupling among the various propagating modes. The greater the coupling, the greater the probability that the wave energy will be more uniformly distributed among all of the possible modes. As a result, the time it takes for all of the wave energy to traverse the entire length of the waveguide is more nearly the same. Thus, whereas a pulse of wave energy simultaneously launched into an ideal guide in two different modes would arrive at the output end of the guide as two pulses, separated in time by an interval proportional to the length of the guide, the same pulse propagating along a guide in which mode coupling is deliberately enhanced would arrive as a single, slightly broadened pulse.
In the case of optical fiber waveguides, the controlled mode coupling imperfections typically include spatially periodic perturbations of the cross-sectional dimensions of the fiber, of the refractive index of the fiber, and of the straightness of the fiber axis. The techniques proposed heretofore for achieving these imperfections in optical fibers can be divided generally into two classes. In the first class, the desired imperfections are introduced during or after fabrication of the fiber using some sort of external arrangement to perturb the fiber parameters. For example, in U.S. Pat. No. 3,687,514, imperfections are illustratively introduced as the fiber is drawn from an enlarged preform by modulating the velocity at which, or the direction through which, the fiber is drawn. One or more piezoelectric members selectively energized by input signals from a filtered noise source are used to move the preform selectively to produce diameter changes or changes in the direction of the fiber axis. Also, the copending application of H. M. Presby, Ser. No. 479,820, filed June 17, 1974 and assigned to the assignee hereof, the diameter and axial alignment of an optical fiber are varied by means of one or more gas streams directed against the fiber as it is being drawn and while it is still relatively molten.
In the second class of techniques, the desired imperfections are introduced into the fiber preform itself before the fiber is drawn therefrom. Examples of this second class of techniques are described in the copending application of D. Marcuse, Ser. No. 522,435, filed Nov. 11, 1974 and assigned to the assignee hereof. Generally, in these techniques, one programs the preform doping process or the preform material deposition process so as to yield perturbations in the preform refractive index or cross-sectional size. The fiber, upon being pulled from such a preform, inherently includes mode coupling perturbations.